More rewriting: ConfigureRules, NewServer.

* The ConfigureRules call is pulled out of devproxy, since CR() belongs
  in package `main` but devproxy is library level.
* DevProxy is replaced by NewServer, to be visible as
  `devproxy.NewServer()` to library callers.
* devproxy.go doesn't concern itself with your command line flags
  anymore.
* NewServer returns a new proxy instance (http.Handler type), instead of
  invoking http.ListenAndServe directly--no more app logic here.
* NewServer takes the ruleset first, then a verbosity bitfield from an
  int.
Split main & devproxy packages.

* Command layer in `devproxy`
* Library layer in the root
* Update references to library layer in command layer
Update build instructions in README.